The present invention relates to a process for producing amorphous magnetically soft bodies with use of a glass of low softening point as a binder and also as an insulator.
It is known that amorphous magnetically soft alloys exhibit more excellent characteristics than crystal materials in respect of corrosion resistance, wear resistance, strength, magnetic permeability, etc. These alloys are used as magnetic materials, for example, for magnetic cores of various devices for use in electric or electronic appliances.
The amorphous magnetically soft alloy is generally in the form of a thin strip, thin wire or powder because of the reasons involved in the quenching process for assuring the amorphous state. Accordingly, when members of specified shape are to be obtained with use of such an alloy in the form of a thin strip or wire, the alloy needs to be pulverized into a powder first and then pressed as heated at a predetermined temperature into bodies.
The powder of amorphous magnetically soft alloy needs to be formed into a body at a temperature lower than the crystallization starting temperature of the alloy so as to retain the amorphous state. However, the alloy powder can not be bulked at this temperature. Amorphous magnetically soft bodies are therefore produced by mixing a glass powder of low softening point with the alloy powder to obtain a material powder, filling the material powder into a hot-forming die, forming the material powder hot at a temperature higher than the softening point of the glass but lower than the crystallization starting temperature of the alloy powder to join the alloy particles to one another with the glass as softened and serving as a binder.
When the material powder as filled in the die is heated to the predetermined forming temperature, the material in the form of a powder has many voids between the particles, and is therefore small in overall thermal conductivity and liable to have a great temperature difference between the material portion adjacent to the wall of the die and the material portion in the center thereof. To heat the material powder uniformly for forming, the powder must be heated for about 20 to about 40 minutes, hence lower productivity.
Further when to be made into an amorphous magnetically soft body having varying wall thicknesses, the powder becomes uneven in temperature owing to the differences in wall thickness, failing to afford a body of uniform characteristics.
On the other hand, an attempt to heat the die to a higher temperature in order to give the powder an increased quantity of heat, shorten the heating time and achieve improved productivity, a still greater temperature difference occurs between the material portion close to the die wall and the material portion in the center, consequently entailing the problem that the temperature of the former portion exceeds the crystallization starting temperature of the alloy when the latter portion reaches the forming temperature to impair the amorphous property.
An object of the present invention is to make it possible to produce an amorphous magnetically soft body merely by heating a body which is formed by cold pressing.
To fulfill the above object, the present invention produces an amorphous magnetically soft body by preforming a material powder into a body first, and heating the preformed body without pressing.
Stated more specifically, an amorphous magnetically soft body is produced from a material powder comprising a powder of an amorphous magnetically soft alloy, a glass having a softening point lower than the crystallization starting temperature of the alloy and a binding resin, by pressing the material powder in a preforming die to prepare a preformed body by the binding property of the resin, and firing the preformed body without pressing at a temperature higher than the softening point of the glass and lower than the crystallization starting temperature of the alloy to join the particles of the alloy with the glass.
The material powder is pressed as placed in the preforming die and thereby consolidated with the binding resin into a preformed body.
The preformed body obtained is heated without pressing, whereby the binding resin is evaporated off, and the glass is softened to join the particles of the amorphous magnetically soft alloy with the glass. According to the present invention, the preformed body is prepared first which is more compact than powders and therefore has a higher thermal conductivity. Consequently, even if heated at an increased rate of rise of temperature, the preformed body can be maintained at a uniform temperature in its entirety without becoming overheated locally.
The preformed body need not be pressed during heating, and therefore need not be placed into a die but can be heated directly in a furnace. Thus, the production process of the invention achieves improved productivity and assures mass production. Furthermore, the binding resin can be evaporated off more effectively to remain in the body in a smaller amount than when the preheated body is heated as placed in a die.